Parosteal lipomas: a new perspective.

Parosteal lipomas, benign adipose tissue tumors situated directly on bone cortex, are unusual neoplasms that appear to emerge from multidirectional mesenchymal "modulation" within the periosteum. These tumors have been described as "periosteal lipomas", "chondrolipomas of soft tissue" and "lipomas of nerves" but they are most commonly believed to originate from the periosteum. Although over 100 of such tumors have been described in the literature, they have not been the subject of a comprehensive review, nor their potential for chondroid modulation and enchondral ossification emphasized. A review of 14 parosteal lipomas from the Bone Tumor Registry, Armed Forces Institute of Pathology, indicates these tumors are frequently associated with chondroid and/or osseous modulation, which permits subclassification into 4 distinct variants. Each of the 4 subtypes (I: No Ossification; II: Pedunculated Exostosis; III: Sessile Exostosis; IV: Patchy Chondro-Osseous Modulation) is illustrated to demonstrate the morphologic basis for radiologic/pathologic correlation and subclassification. A brief overview of the literature and pathogenesis of this unusual lesion is presented and discussed.